1. Field of the Invention
The present invention relates to a spark plug used for igniting an internal combustion engine.
2. Description of the Related Art
The metallic shell of a spark plug is fixedly attached to an insulator by means of crimping. Specifically, the insulator is inserted into the metallic shell formed into a tubular shape, and then by use of dies a compressive load is applied to the peripheral edge of a rear end portion (a portion to be crimped) of the metallic shell. By this procedure, the portion to be crimped is curved toward a flange-like protrusion formed on the outer circumferential surface of the insulator to thereby become a crimped portion, whereby the insulator is fixed in place. The metallic shell is generally formed from a steel material such as carbon steel.
A method for firmly joining the insulator to the metallic shell by means of the crimped portion is specifically carried out in the following manner. As shown in FIG. 2(a), when a portion-to-be-crimped 1dxe2x80x2 is axially compressed by means of crimping dies 110 and 111, the portion-to-be-crimped 1dxe2x80x2 is plastically deformed radially inward in a compressed condition. Packings 60 and 62 and a filler material 61 such as talc are usually disposed between the portion-to-be deformed 1dxe2x80x2 and a flange-like protrusion 2e (in some cases, the filler material may be omitted, with only a single thick packing disposed). When compressive deformation of the portion-to-be-crimped 1dxe2x80x2 increases, a load begins to be imposed on the packings 60 and 62, the filler material 61, and the flange-like protrusion 2e (hereinafter, these are generically and collectively called a xe2x80x9cportion to be compressedxe2x80x9d). While the portion to be compressed undergoes compressive deformation, plastic deformation of the portion-to-be-crimped 1dxe2x80x2 proceeds further. Then, as shown in FIG. 2(b) which is a step following the step shown in FIG. 2(a), when a final value for a compression stroke for crimping is reached, unloading is performed to thereby complete the crimping process (the portion-to-be-crimped 1dxe2x80x2 becomes a crimped portion 1d). The unloading induces some springback of the crimped portion 1d. However, since the crimped portion 1d is plastically deformed, the crimped portion 1d retains the compressed portion in an elastically deformed condition, thereby inducing a fastening force for firmly joining the insulator 2 to the metallic shell 1.
3. Problems Solved by the Invention:
Along with a recent tendency of an engine toward complex arrangement around heads and an increase in valve diameter, spark plugs show a marked tendency towards a decrease in diameter and increase in length. However, decreasing the diameter of a spark plug requires employing a metallic shell having a small diameter and a thin wall. As is apparent from the above-described principle, a force for fastening the insulator against the metallic shell is induced by reaction from the crimped portion 1d. Since a reduction in the diameter and wall thickness of the metallic shell is accompanied by a reduction in the cross-sectional area of the crimped portion 1d, bringing stress arising on the cross section of the crimped portion 1d to the same level as a conventional one requires a reduction in compression stroke for crimping. Thus, total fastening force decreases by an extent corresponding to the reduction in the cross-sectional area. As a result, gastightness established between the metallic shell and the insulator is deteriorated. Particularly, when harsh vibrations act on a spark plug as in high-speed, high-load driving, crimping of the spark plug may be loosened, and thus gastightness is more likely to be deteriorated.
By contrast, an attempt to maintain the total fastening force at the same level as a conventional one involves an increase in stress by an extent corresponding to a decrease in the cross-sectional area of the crimped portion 1d; as a result, the strength of the crimped portion 1d fails to endure the stress, thereby leading to a failure to maintain gastightness.
An object of the present invention is to enable, in a spark plug configured such that a metallic shell is joined to an insulator through crimping, the metallic shell to be firmly joined to the insulator by means of a sufficient fastening force even when the diameter of the spark plug is reduced, to thereby enhance gastightness and vibration resistance.
The above object of the present invention is achieved by providing a spark plug comprising a rodlike center electrode, a rodlike insulator surrounding the center electrode and having a protrusion at a central portion thereof, a metallic shell assuming an open-ended, tubular shape and surrounding the insulator, and having two protrusions and a thin-walled portion formed on an outer surface thereof at a central portion thereof with respect to the direction of said axis, the thin-walled portion being located between said two protrusions and being thinner than said two protrusions; and a ground electrode facing the center electrode and defining a spark discharge gap in cooperation with the center electrode, and characterized in that:
an insulator insertion hole into which the protrusion of the insulator is inserted is formed in the metallic shell while extending in the direction of an axis (O); when a side toward the spark discharge gap with respect to the direction of the axis is taken as a front side, a rear end portion of the metallic shell is crimped by a cold crimping step toward the insulator to form a curved, crimped portion; and, in order to achieve the above object,
the inside diameter of the insulator insertion hole of the metallic shell is 8-12 mm as measured at a position where the inner wall surface of the insulator insertion hole transitions to the inner wall surface of the crimped portion with respect to the direction of the axis of the metallic shell; and the cross-sectional area S of the metallic shell as measured when the metallic shell is cut at the position by a plane perpendicular to the axis, and the carbon content of a steel material used to form the metallic shell satisfy either of the following conditions A and B:
condition A: 15xe2x89xa6S less than 29 mm2 and a carbon content of 0.20%-0.50% by weight; and
condition B: 29xe2x89xa6S less than 35 mm2 and a carbon content of 0.15%-0.50% by weight.
When a side toward a spark discharge gap with respect to the direction of the axis is taken as a front side, a tool engagement portion (a so-called hexagonal portion) is usually formed on the metallic shell of the spark plug to be located adjacent to and on the front side of the crimped portion of the metallic shell. When the spark plug is to be mounted into a plug attachment hole formed in an internal combustion engine, a tool such as a wrench is engaged with the tool engagement portion. Conventionally, the tool engagement portion of a spark plug has dominantly employed an opposite side-to-side dimension of 16 mm or more, so that the cross-sectional area of the crimped portion can be 40 mm2 or more. However, the previously mentioned tendency to decrease the diameter of a spark plug is also bringing about increasing demand for reducing the size of the tool engagement portion, for, for example, the following reasons: employment of a direct ignition method-in which individual ignition coils are directly attached to upper portions of corresponding spark plugs-narrows an available space above a cylinder head; and the previously mentioned increase in area occupied by valves forces a reduction in the diameter of plug holes. As a result, the opposite side-to-side dimension of the tool engagement portion is forced to be reduced to, for example, 14 mm or less from a conventionally available dimension of 16 mm or more. Condition A or B of the present invention provides the range of the cross-sectional area of the crimped portion in view of employing a metallic shell whose diameter is reduced such that the opposite side-to-side dimension of the tool engagement portion is not greater than 14 mm, for example. Also, the range of the inside diameter (8-12 mm) of the insulator insertion hole of the metallic shell is determined in view of a reduction in the diameter of the metallic shell. Notably, the inside diameter of the insulator insertion hole of the metallic shell is that measured at a position where the protrusion of the insulator is inserted.
A feature of the present invention is to form the metallic shell whose crimped portion has a cross-sectional area as reduced as mentioned above, from a steel material whose carbon content is increased according to the cross-sectional area, so as to impart to the crimped portion strength capable of sufficiently enduring an increased fastening stress. As a result, the metallic shell can be firmly joined to the insulator by means of a sufficient fastening force, thereby enhancing gastightness and vibration resistance.
Specifically, the outside diameter of the metallic shell is classified into two categories, or condition A and condition B, according to the range of the cross-sectional area S of the crimped portion. Condition A employs the following range of the cross-sectional area S of the crimped portion: 15xe2x89xa6S less than 29 mm2. In this case, the carbon content of a steel material used to form the metallic shell is selected so as to fall within the range of 0.20% by weight to 0.50% by weight. Condition B employs the following range of the cross-sectional area S of the crimped portion: 29xe2x89xa6S less than 35 mm2. In this case, the carbon content of a steel material used to form the metallic shell is selected so as to fall within the range of 0.15% by weight to 0.50% by weight.
In either case, when the carbon content of a steel material falls below the lower limit, the strength of the crimped portion becomes insufficient to endure a fastening stress, thereby leading to lack of gastightness or vibration resistance. By contrast, when the carbon content of a steel material exceeds the upper limit, in the case of a metallic shell to be manufactured by a cold forging (press-forming) process, deformation resistance of the steel material becomes excessively high, thereby leading to a reduction in working efficiency or a reduction in the life of a working tool and thus to an increase in manufacturing cost. This tendency is particularly marked in the case of a metallic shell having a small diameter and a long axial length.
Condition A, which employs a narrower range of the cross-sectional area S of the crimped portion, sets a higher lower limit for the carbon content of a steel material, since greater stress is required than in the case of condition B, in order to secure gas-tightness. Condition A also requires at least 15 mm2 for the cross-sectional area S, since a metallic shell having a small diameter such that the cross-sectional area S of the crimped portion is less than 15 mm2 fails to maintain gastightness. This also applies to the lower limit (8 mm) of the inside diameter of the insulator insertion hole of the metallic shell.
The above-mentioned crimped portion can be formed by means of cold crimping. Cold crimping has an advantage of employing simple crimping equipment and thus having a short cycle time, which is efficient.
Next, an anticorrosive film is formed on most conventional types of metallic shells for spark plug use and formed from a carbon steel or the like. Galvanization, which is inexpensive and excellently anticorrosive, has been employed as a method for forming the anticorrosive film. However, in the case of the metallic shell used in the present invention and formed from a steel material of high carbon content, galvanization raises the following problem.
In electrogalvanization, zinc, which is more basic than iron, must be deposited on the surface of iron; therefore, electric potential for galvanization is set relatively high. As a result, hydrogen tends to be generated in the process of galvanization. The thus-generated hydrogen is absorbed into a base material, or a steel material. However, in the case of a high-strength steel material, the thus absorbed hydrogen is known to tend to cause hydrogen embrittlement; i.e., a high-strength steel material tends to become brittle as a result of absorption of hydrogen. The presence of restraint stress induced from tension is known to play an important role in occurrence of hydrogen embrittlement. The crimped portion of the metallic shell is subjected to tensile stress at all times in order to endure fastening stress and is thus likely to suffer hydrogen embrittlement.
In any case, when crimping is loosened as a result of hydrogen embrittlement, the gastightness and vibration resistance of the metallic shell are impaired. Hydrogen embrittlement fracture is known not to occur immediately upon establishment of embrittlement conditions (i.e., absorption of a certain amount or more of hydrogen and imposition of restraint stress), but to occur after a certain incubation period. Such fracture is also called delayed cracking or delayed fracture.
The spark plug of the present invention uses a steel material whose strength is enhanced through an increase in carbon content, as mentioned above. Since such a steel material is highly susceptible to hydrogen embrittlement, the crimped portion must be designed so as to prevent occurrence of hydrogen embrittlement. The higher the restraint stress, the shorter the incubation period of delayed fracture. Therefore, delayed fracture is more likely to occur in a spark plug which, in order to compensate for a reduction in the cross-sectional area of the crimped portion, employs crimping of a long compression stroke so as to increase fastening stress. When cold crimping is employed, hydrogen embrittlement is likely to occur at a part of the crimped portion where stress concentrates due to work strain, and employing a long compression stroke increases the amount of accumulated work strain.
When galvanization is to be applied to the metallic shell of the spark plug of the present invention, the galvanization conditions must be carefully determined so as to prevent excessive generation of hydrogen in the process of galvanization. However, narrowing galvanization conditions encounters difficulty in controlling the conditions, thereby leading to increased cost.
Thus, preferably, a nickel plating layer is employed in place of conventional galvanization, for use as an anticorrosive film to be formed on the metallic shell. In contrast to zinc, nickel is more noble than iron; thus, nickel can be deposited smoothly without the need to increase electric potential for electrolytic nickel plating. Therefore, nickel plating, by nature, is unlikely to involve generation of hydrogen and thus unlikely to raise a hydrogen embrittlement problem.
In the claims appended hereto, reference numerals assigned to elements are cited from the accompanying drawings for providing fuller understanding of the nature of the present invention, but should not be construed as limiting the invention.